1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly, to components for storing and transporting the liquid fuel and reactants for use in such fuel cells.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel-processing is expensive and requires significant volume, reformer based systems are presently limited to comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically conductive, electronically non-conductive membrane (PCM). Typically, a catalyst which enables direct oxidation of the fuel on the anode is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and oxygen molecules involved in the cathodic reaction and travel through a load, providing electrical power.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, methanol, typically in an aqueous solution is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental reactions that occur in a DMFC which allow a DMFC system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate (more specifically, failure to oxidize the fuel mixture will limit the cathodic generation of water, and vice versa).
Fuel cells and fuel cell systems have been the subject of intensified recent development because of their ability to efficiently convert the energy in carbonaceous fuels into electric power while emitting comparatively low levels of environmentally harmful substances. The adaptation of fuel cell systems to mobile uses, however, is not straight-forward because of the technical difficulties associated with reforming most carbonaceous fuels in a simple, cost effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for substantially pure hydrogen (which is a gas under the relevant operating conditions), presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy absorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol or an aqueous methanol solution (and to a lesser extent, ethanol, propane, butane and other carbonaceous liquids or aqueous solutions thereof).
In particular, DMFCs are being commercially developed for use in portable electronic devices. Thus, the DMFC system, including the fuel cell, and the components may be fabricated using materials that not only optimize the electricity-generating reactions, but which are also cost effective. Furthermore, the manufacturing process associated with those materials should not be prohibitive in terms of labor intensity cost.
As noted, typical DMFC systems include a fuel source, fluid and effluent management systems, and a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, and a membrane electrode assembly (“MEA”) disposed within the housing. A typical MEA includes a centrally disposed protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is Nafion® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane based on perflouorocarbon polymers with side chain termini of perflourosulfonic acid groups, in a variety of thicknesses and equivalent weight. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer functions to evenly distribute the liquid fuel mixture across the anode in the case of the fuel, or the gaseous oxygen from air or other source across the cathode face of the PCM. In addition, flow field plates are often placed on the surface of the diffusion layers which are not in contact with the coated PCM. The flow field plates function to provide mass transport of the reactants and by products of the electrochemical reactions, and they also have a current collection functionality in that the flow field plates act to collect and conduct electrons through the load.
Many direct methanol fuel cell systems employ an active management scheme to manage the reactants and byproducts in the fuel cell, including pumping or otherwise causing the fuel mixture to be transported to the anodic face of the PCM. In addition, there may be an actively managed system which removes anodically evolved carbon dioxide from the anode face of the PCM, or which induces air to the cathode face of the PCM. To increase the utility and effectiveness of DMFC systems, there may be a need for a variety of types of diffusion layers and flow field plates. In some cases, a hydrophobic (or partially hydrophobic) diffusion layer is a useful component to assist in the control of reactants or byproducts.
Some of these active transport mechanisms however can be costly, both in terms of the components required and the complexity introduced into the manufacturing of such mechanisms. This reduces the feasibility for these items to be produced on a commercial scale. In addition, adding additional components, such as pumps and other active transport devices, can increase parasitic power losses in an already small device. Such components further add volume to a system that must meet demanding form factors.
It is also noted that, when fuel from a fuel source or reservoir builds up on the anode side of the MEA, any extra fuel not consumed in the reaction that may pass from the anode aspect of the MEA to the cathode aspect of the MEA through the membrane and be lost to that reaction. One method of preventing this reaction is to circulate fuel through the anode chamber, but this requires the use of active transport components. On the cathode side, the cathode diffusion layer can become saturated by water (a byproduct of the cathode half reaction), that has passed through the membrane, as well as water that is generated by the cathodic oxidation of the methanol that has crossed over the membrane. Thus, water builds up in the cathode chamber of the fuel cell. The cathode diffusion layer can thus become flooded, in which case the cathode half reaction is compromised, or even halted. In either case, whether it is fuel loss on the anode side or cathode flooding, the half reactions at either the anode or the cathode can correspondingly be compromised or even prevented, thus reducing the efficiency of the fuel cell.
Typically, the risk of cathode flooding has been mitigated by encouraging cathodic airflow across the cathode diffusion layer to remove water from the cathode layer. This, however, increases the cost and complexity of the fuel cell system, thus adding to the expense of manufacture, as well as introducing the above-mentioned parasitic losses. There have been attempts to reduce the risk of cathode flooding by providing active drying of water from the cathode chamber, but this can increase the cost and complexity of the fuel cell, adding to the expense of manufacture, as well as introducing additional possibility of parasitic losses.
There remains a need, therefore, for a fuel delivery cartridge, direct methanol fuel cell, and a direct methanol fuel cell system that provides optimal fuel delivery properties by which fuel can be delivered to the active anode chamber as it is consumed, thereby minimizing the amount of additional fuel that is introduced, and limiting the amount of fuel that crosses over the membrane. There remains a further need to provide a cathode chamber in the fuel cell that resists water build up, while allowing oxygen to come into contact with the cathode face of the membrane.
It is thus an object of the invention to provide a fuel cell and fuel cartridge that provides fuel to the anode as it is consumed and reduces the risk of cathode flooding, while keeping the cost and complexity of the fuel cell to a minimum.